1. Field of the Invention
This invention relates generally to the forestry industry, and more specifically to the field of wood working machines of the type that are used to process wood to form chips or wafers for pulp or waferboard production, to form rough or finished lumber, or to form veneer for the production of plywood or laminated veneer lumber. Most particularly, this invention relates to wood working knives used in such machines, and to clamping assemblies to hold the knives in place.
Wood is an important natural resource that is used in many of today's modern products. Within the forest industry, trees are harvested, cut into logs, and then subsequently undergo various processes to transform the logs into finished products. For example, in the pulp or oriented-strand board industries, the logs are passed through a machine which turns the solid log into chips or wafers. Such machines are typically referred to as chippers, which may be in a disc or a drum form, and waferizers or stranders, which can also take a number of forms.
Within the sawmill industry, it is common for logs or semi-manufactured lumber to be passed through machines which chip away the outside portions of the wood being processed to form rough lumber and a multitude of wood chips. Such machines are commonly referred to as chipper canters, chipper edgers and chipper slabbers, each of which can take a variety of different forms. Typically, this rough lumber is then processed by planers to yield finished lumber having a smooth cut surface and wood shavings as a by-product.
Within the veneer industries, logs are turned on lathes to form veneer sheets that are subsequently used for the manufacturing of plywood or laminated veneer lumber. Such machines are commonly referred to as veneer lathes.
Typical of planers, chippers, waferizers and other such wood processing machines is that they carry a number of knives mounted to a moving base, such as, for example, a rotating disc or drum. The wood being processed is moved into the path of the rotating knives and the blade contacts the wood at a depth and orientation that results in the formation of wood chips, shavings wafers, or strands. With chipper edgers, chipper canters, planers, or other similar wood finishing devices, the knives are also appropriately positioned so as to result in the formation of a cut or planed surface on the wood being worked. With veneer lathes however, the knife remains relatively stationary while the log, rotated about its axis, is engaged by the knife.
Common to all the aforementioned machines is that the repetitive contact between the cutting edge of the knife and the wood causes the cutting edge to wear and become dull over time. When the knife becomes too dull, it ceases to cut the wood cleanly and effectively. For example, in chippers, waferizers, and veneer lathes, a dull knife results in chips, wafers, or veneer of reduced quality and/or inconsistent size. In chipper canters, edgers, slabbers, or other like machines where rough or finished lumber is produced, knife sharpness influences the quality and accuracy of the finish of the wood being processed.
Traditionally, the method for maintaining knives sharp in the machines has been to remove the knife from the knife clamping assembly within the machine, sharpen the knife by regrinding it, and then replace the knife in the clamping assembly. However, this approach suffers from a number of known limitations. During each regrinding, portions of the knife must be ground away to create a fresh sharp cutting edge. This regrinding results in a change in size of the blade that if left unadjusted, would result in an altered location of the cutting edge after each regrinding. Specifically, the position of the cutting edge is altered relative to the features that locate the knife in the clamping assembly.
The result is that the position of the cutting edge can be displaced from its desired and intended location relative to the wood being worked or important associate components within the machine such as anvils and guide plates. Unless the position of the knife is adjusted in the clamping assembly each time, which is difficult to do accurately and is also time consuming, the performance achieved with the machine is degraded, sometimes to unacceptable levels. For example, with chipper canters, a precise positioning of the face or finishing knives relative to the wood being processed is a requirement for an accurate cut surface. Relatively small deviations in position can have a measurable impact on the quality of the finish achieved.
Another limitation of this approach is that the grinding may not be sufficiently precise. Equipment utilized within wood processing facilities is often such that accurate form (shape and angle) of the cutting edge cannot be maintained. Furthermore, during the on-site regrinding, the knives are sometimes damaged, whether through overheating or other grinding process irregularities. This can reduce the quality of the cutting edge causing the knife to wear faster degrading performance. Similarly, deviations in the form of the cutting edge can also result in a reduction in performance.
To overcome such problems, it has become common to use disposable blades, most often of a reversible, or double-edged, design. Such a knife is shown, for example, in U.S. Pat. No. 4,047,670 issued Sep. 13, 1977 to Aktiebolaget Iggesunds Bruk. The knife is essentially a planar, elongate body with one cutting edge running along one side of the elongate body and a second cutting edge running along the other. The knife is mounted in a knife clamping assembly that is sized and shaped to secure the blade during operation and allow for easy and rapid knife changes. In use, when the first cutting edge becomes dull, the knife is reversed and the second cutting edge is presented and used. When that cutting edge has also become worm, the knife is disposed of and replaced with a new one having two more fresh cutting edges.
With disposable designs, the problems relating to the grinding of the knife are eliminated because the knives are not reground. The dimensions and form of the knife, controlled by the knife manufacturer, remain unaltered between changes. There is also a certain gain in efficiency, because the smaller lightweight disposable blades, typically of higher quality materials and manufacture, allow for increased run times between changes. Also, because of the ease of replacing and rotating the knives, machine stoppages for knife maintenance is further reduced.
However, this solution also has some drawbacks. In some applications, the amount of cutting edge wear that affects performance can be quite minimal. Under such circumstances, the amount of regrinding that is required to restore the cutting edge is quite small such that the edge may only need to be lightly refreshed. In these situations, many of the profiles of the disposable blades lack an efficient and cost-effective method for restoring the cutting edge without significantly altering the shape and position of the cutting edge upon reinstallation into the clamping assembly.
Another problem that affects knives used in many types of wood processing machines is the difficulty in securing the knives in the clamping assemblies under the action of the cutting forces. The problem is most prevalent with disposable blade designs where the requirement for cost effectiveness and competitiveness mandates that the blades be compact and lightweight. Such compact blades are often difficult to secure in the clamping assembly such that they can resist the various types of loads encountered across the different types of applications. Chipping applications, for example, involve significant cutting forces directed towards the underside of the knife whereas with planers or waferizers, these cutting forces are relatively low. With chipper edgers and chipper canters, the face or finishing knives can often encounter significant loads directed to the topside of the cutting edge.
One particular problem that affects knife designs is the unsymmetrical nature of the loads distributed along the knife length. Wood is not a homogeneous material. Sometimes, the wood will exert a greater force against one localized area of the cutting edge than against the remainder of the blade. The most common reason for this is that the cutting edge strikes a knot or some other irregularity in the wood. Further, with some arrangements, one or both ends of the knife may utilized to produce a side cut. This can add to the non-symmetric nature of the loads encountered by the knife.
In such situations, twisting may occur. Typically, when the knife twists, the portion of the edge in contact with the irregularity bears a greater force. This difference in force along the length of the blade creates a torque on the knife which, if sufficiently large, can cause the knife to displace or twist in its mounting. A problem is to provide a knife and mounting assembly which is capable of handling such twisting forces.
Another consideration is the relationship between the design of the knives and their mountings, and the quality of the wood product they produce. Specifically, the quality of the end product is dependent on the accuracy of position of the cutting edge relative to the machine achieved during the initial installation, and subsequently, the ability to maintain the position when subjected to load. The greater the accuracy of the knife position, in general, the better the quality of the wood working results.
In most knife arrangements, knives are inserted into the clamping assembly by hand. Under such circumstances, precise positioning may be difficult, simply because the required precision may be greater than is possible in a manual operation. In many cases, the knives are changed in situations that are physically awkward for the person changing the knife. Depending on the circumstances, the person may need to reach overhead or around cumbersome components to perform the change. This renders precise positioning even more difficult.
Further, with many designs a range of position often exists within the clamping assembly in which the knife can be secured. This range of position, although often limited to a degree, allows for a variation in the location of the cutting edge relative to the wood being worked. One approach to overcoming this limitation is shown in U.S. Pat. No. 6,058,989 granted to Iggesund Tools AB. This approach is to employ a biasing element within the knife assembly itself to bias the knife into a predetermined position within the cassette to increase the accuracy of position of the cutting edge relative to the machine. However while helping to maximize the accuracy of position of the cutting edge during initial installation, this approach does not minimize the chances for subsequent displacement when subjected to load.
Another issue in this field is the requirement for many different clamping assemblies and knives for the many different types of wood working applications. For example, many wood working applications have different dimensional requirements relating to the knife and the clamping assembly. Thus, different applications may impose different requirements for the shape of the adjacent clamping components. For example, some applications may require that a substantial distance be present between the cutting edge and the lower clamping component (that which is in contact with the underside of the knife). This may be necessary to avoid undesired damage to the cut particles produced or to allow for sufficient space to permit for unobstructed wood movement. Other applications may require the close presence of the lower clamping component to function as a deflecting surface for the proper formation of wafers, veneer, or to intentionally breakup cut particles.
Similarly, strength requirements also differ between applications or according to the type of species being processed, climatic factors, or other external variables. This imposes further restrictions on the size and shape of the knife and the surrounding clamping components since it requires that they be designed to be able to sustain the loads encountered within the relevant geometric constraints.